It sounds as though you already downloaded and installed the plugin as described in this article: How do I watch Flash videos with Firefox for Android?

I look for a thread about the Note 4 and found confirmation that it "should" work here: Note 4 Adobe Flashplayer - Android Forums at AndroidCentral.com.

However, I don't have that device so you might try that forum if you can't get it working.

Are you still using Android 4.4.4 KitKat or did you upgrade to 5.0 Lollipop?

It appears that Flash is no longer working for a number of people with various Android versions. See https://support.mozilla.org/en-US/questions/1038039

It should be fixed now https://bugzilla.mozilla.org/show_bug.cgi?id=1112086#c23

Note that Bugzilla is Not a discussion forum like here.

However if you are using Android 5.0 Lollipop it may or may not still work